1. Dry Transfers
Dry Transfers typically consist of a carrier sheet bearing transfer elements such as symbols, characters or other discrete shapes of film and low-tack adhesive on the film shapes. Dry transfers are distinguished from other transfers which require water or other solvent or heat for releasing the transfer elements from the carrier sheet and for adherence of the transfer elements to the receiving surface. U.S. Pat. No. 3,013,917, issued Dec. 19, 1961, to M. Karlan et al, and U.S. Pat. No. 3,131,106 issued Apr. 28, 1964 to F. W. McKenzie disclose typical dry transfers. During nearly two decades, dry transfers have been produced by silk-screening plural areas of film in discrete shapes, especially symbols or characters, on a common carrier sheet having release properties, and silk-screening or otherwise coating low-tack adhesive on the film shapes. It is impractical in production to depend on precisely registering or aligning the outlines of the film shapes and the outlines of the adhesive, and therefore it has been usual practice to have the adhesive extend beyond the film shapes as "overlap" onto the carrier sheet. The overlap adhesive has the desirable purpose of retaining the film shapes in position on the carrier sheet, guarding against premature separation in case the sheet is subjected to such rough handling as to break the weak bond between the film shapes and the carrier sheet.
Dry transfers are used by locating a desired transfer element, adhesive-side down, on a selected area of a receiving surface. The carrier sheet is pellucid, that is, sufficiently transparent for this purpose. The adhesive has such limited tack that it does not adhere on contact, so that the sheet can be moved about to adjust a selected transfer element to exactly the right position on the receiving surface. The selected area of the dry transfer sheet is commonly stroked as with a ball-point pen so that a secure adhesive bond is made between the selected transfer element and the receiving surface. The transfer element is released from the carrier sheet either as a result of the mechanical effect of the stroking operation (due perhaps to greater distortion of the carrier sheet than the film shape) or as a result of a so-called "tug of war" between the newly established adhesive bond to the receiving surface and the weaker bond of the film shape to the carrier sheet.
The adhesive has a stronger bond to the carrier sheet in the overlap areas, around the outlines of the film shapes. There is a tendency of some "overlap" adhesive to transfer to the receiving surface, due to weakness of the adhesive-to-carrier bond, due to ragged tearing of the adhesive around the outline of the transferred film, or due to transfer of part of the thickness of the overlap adhesive. Ball-point pen pressure may develop a strong bond to the receiving surface. Transferred adhesive is exposed on the receiving surface so that it picks up dirt. One effort at eliminating the problem is disclosed in U.S. Pat. No. 3,298,850, issued Jan. 17, 1967 to K. J. Reed et al. It is proposed there that the adhesive should react with the carrier sheet in the overlap areas where they are in contact so as to nullify the adhesive quality. This approach seems to have had only limited success, perhaps due to practical limitations of the processes and materials involved. A second problem connected with the "overlap" adhesive is that where it does adhere securely to the carrier sheet, it tends to secure the outline of the film shape to the carrier sheet and thus it impedes the transfer operation. This problem normally is met by incorporating a so-called "shear agent" into the adhesive. When a layer of adhesive containing a shear agent is bonded to the carrier sheet, it tends to shear readily along the outline of the film shape that is newly bonded to the receptor sheet. In any case, the formulation of the adhesive with proper shear properties and the proper degree of tack and with qualities intended to limit transfer of overlap adhesive complicates the manufacture of dry transfers.